In a known pressure measuring structure, a diaphragm is generally provided perpendicular to the direction in which pressure is applied such that the diaphragm can be finely elastically deformed due to the pressure applied to the diaphragm, and four strain gauges are attached to the top of the diaphragm so as to convert the elastic deformation of the diaphragm into an electrical signal (generally, a difference in voltage displacement). At this time, the strain gauges are attached to the diaphragm such that the strain gauges are provided in a pair symmetrical with respect to the center of the diaphragm. A Wheatstone bridge circuit formed on a printed circuit board (PCB) accurately calculates a pressure value applied to the diaphragm from the electric signal obtained by the strain gauges.
Generally, a pressure sensor may include an input unit, having a diaphragm, to receive external pressure, a first circuit board electrically connected to strain gauges of the diaphragm to receive an electric signal from the strain gauges and processing the received electric signal using a Wheatstone bridge circuit, and a second circuit board electrically connected to the first circuit board, the second circuit board having a circuit device to transmit a pressure value to a controller provided outside the sensor.
Meanwhile, the size of the pressure sensor may be restricted. For example, a pressure sensor may be mounted to an external signal processing (ESP) module used in an antilock brake system (ABS) for automobiles. As is well known, a solenoid valve, a pump line and the like are densely disposed in the ESP module. The ESP module is filled with a high pressure fluid, which is pulsated to perform braking when the ESP module is driven. The components of the ESP module are disposed as densely as possible so as to reduce the size of the device. In addition, it is necessary to configure a pressure sensor provided to sense pressure of the fluid in the ESP module such that the sectional area of the pressure sensor is limited.
Such a pressure sensor is schematically shown in FIG. 7, which is a sectional view illustrating a known pressure sensor.
A diaphragm 221 is disposed at the upper end of an input unit 220, which is coupled to the lower end of a cylindrical casing 210. Four strain gauges 222 are attached to the top of the diaphragm 221. A socket 223, made of metal, is coupled to the upper part of the input unit 220 such that the socket 223 surrounds the input unit 220. A first circuit board 230 having a Wheatstone bridge circuit mounted therein is securely coupled to the upper end of the socket 223.
The strain gauges 222 are electrically connected to the first circuit board 230 by wire bonding such that influence on elastic deformation of the diaphragm 221 is minimized. In this case, the distance between the top of the diaphragm 221, to which the strain gauges 222 are attached, and the first circuit board 230 is restricted such that the distance does not exceed a predetermined value, i.e., such that welding portions are not separated from opposite ends of wires 240, which are inclined, when the relative position between the strain gauges 222 and the first circuit board 230 is changed due to elastic deformation of the diaphragm 221.
Meanwhile, a second circuit board 270 is coupled to the upper end of the cylindrical casing 210. Electrode rods 250 disposed in the pressure sensor 200, where the electrode rods 250 protrude upward from the pressure sensor 200, transmit a pressure value to the outside are electrically connected to the top of the second circuit board 270. The second circuit board 270 includes elastic terminals 260 protruding downward to receive the pressure value generated by the first circuit board 230.
The first circuit board 230 is provided at the center thereof with a receiving passageway through which the strain gauges 222 are connected to the first circuit board 230 by wire bonding. It may be assumed that a plurality of electrode rods are mounted around the receiving passageway of the first circuit board 230 in order for the first circuit board 230 to be directly electrically connected to contact points (not shown) of an ESP module. In this case, the distance between the respective electrode rods is great due to the receiving passageway formed at the center of the first circuit board 230, with the result that the electrode rods may not be aligned with the respective contact points formed at the ESP module. For this reason, the second circuit board 270 is further provided between the first circuit board 230 and the contact points of the ESP module such that the first circuit board and the second circuit board are electrically connected to each other through a plurality of electrode rods disposed at great intervals, and electrical connection between the second circuit board and the ESP module using a plurality of electrode rods densely disposed at the center of the second circuit board. In the known pressure sensor, as described above, the receiving passageway is formed in the first circuit board, and therefore, the first circuit board and the second circuit board are necessary.
Also, each of the elastic terminals 260 includes an outer pipe body 261 having a spring (not shown) mounted therein, the outer pipe body 261 fixedly extending through the second circuit board 270, and an inner pipe body 262 disposed in the outer pipe body 261 such that the inner pipe body 262 can elastically slide upward. The outer pipe body 261 and the inner pipe body 262 are made of a conductive material. When the input unit 220 is inserted into the lower end of the cylindrical casing 210, the elastic terminals elastically move upward to contact an electrical pattern (not shown) formed on the first circuit board 230.
The elastic terminals 260 cannot be fixed to the first circuit board 230 because the distance between the top of the diaphragm and the first circuit board is restricted. That is, if the outer pipe body 261 is placed under the first circuit board 230 so as to securely couple the elastic terminals 260 to the first circuit board 230, the distance between the strain gauges 222 and the first circuit board 230 is excessively increased due to the length of the outer pipe body 261 protruding from the lower end of the first circuit board 230, with the result that welded portions of the wires may not be stably maintained during a designed life span.
In the known pressure sensor, therefore, the first circuit board having the Wheatstone bridge circuit and the second circuit board having a circuit device are separated from each other where the second circuit board is disposed above the first circuit board, and the upper and lower circuit boards are connected to each other using the elastic terminals, due to a restricted area of the pressure sensor.
Since the elastic terminals are used in the known pressure sensor, the distance between the first and second circuit boards is increased so as to provide a space necessary for the elastic terminals to expand and contract, with the result that the vertical length of the pressure sensor is increased.
Also, each of the elastic terminals includes a plurality of components, such as the outer pipe body, the inner pipe body, and the spring, such that the result that assembly efficiency is lowered, and manufacturing costs are increased.